PORTLAND — To local businesses, they may seem more like treasure ships than cruise ships.

But four years after the first and only study of Portland’s cruise-ship business, some people wonder about the full economic impact of the industry – and what lies on the horizon.

The 2009 study found that passengers spent an average of $80 per person on goods and services in the Portland region. That average increased to $110 per person when the price of ship-sponsored tours was included.

And because all that spending has a “multiplier” effect, it was estimated that cruise ship visits contribute between $5.8 million and $8 million annually to the local economy, and supported between 69 and 96 full- and part-time jobs.

The study, funded by the city and conducted by University of Maine economics professors Todd Gabe and James McConnon, surveyed 1,300 passengers aboard seven ships that docked in Portland during the fall of 2008.

But a lot has changed since 2008, when a total of 35 ships called at Portland, bringing about 48,000 passengers.

The number of visiting ships and passengers initially soared, hitting all-time highs of 71 ships in 2010 and more than 85,000 passengers in 2011. Since then, however, the port’s popularity has stalled. This year, 58 cruise ships are expected to visit, bringing an estimated 71,000 passengers.

There are many possible reasons for the falloff, experts say, such as the lingering economic recession or several high-profile incidents involving cruise ships. They include the Costa Concordia, which capsized last year in Italy, killing 32 people.

The cruise industry itself has changed too, according to City Hall spokeswoman Nicole Clegg.

New cruising ports have opened in Asia and elsewhere, inviting cruise lines to redeploy their ships and sometimes skip voyages to Canada and New England. “There’s been a reshuffling of the deck,” Clegg said.

But another change in the industry may ultimately benefit the city.

Cruise lines are building bigger ships, a trend that could deliver more potential customers to Portland, Clegg said.

In 2015, the city will receive its largest passenger liner ever, the 4,180-passenger Quantum of the Seas. The Royal Caribbean line ship, launched in April, will tie up at Ocean Gateway Pier II, the deep-water “megaberth” that opened in 2011.

This year, the largest cruise ship visiting Portland is expected to be a sister ship, Explorer of the Seas, which will bring a projected 3,114 passengers in September.

Besides that trend, more small ships are also stopping in Portland.

In 2010, the city became home port for the 98-passenger Independence and the 49-passenger American Glory. Together with several other vessels of fewer than 500 passengers, these modest-sized vessels account for more than half of the 58 cruise stops expected this year.

It’s hard to estimate the economic impact of these frequent visitors, since the 2009 study looked only at ships that each carried around 2,000 passengers. But while their numbers are comparatively few, travelers aboard small ships are often highly affluent; small-ship fares are typically three or four times the price of those for larger vessels.

As a result, study author Gabe said, the economic impact of a passenger aboard a small ship may be greater than estimated.

“We know that, in general, passengers with higher incomes spend more, so if anything you’d expect more spending in Portland,” he said.

On Monday, some of the nearly 3,000 passengers returning to the Carnival Glory were bullish about Portland.

Mary Mayfield, of New Jersey, said she had bought T-shirts, a cap, framed photographs and a map of Maine while browsing Old Port shops. Together with lunch for her and a friend, she estimated she spent about $90.

“I’ve never been to Portland before, and I didn’t have a shopping list in mind, but it’s easy to go through a lot of money when there are so many stores close by,” she said. “We had a good day, so I guess this was a good investment.”